Reconstructing the Evolutionary History of Dissimilatory Sulfur Cycling Genes

The sulfur cycle plays an important yet understudied role in regulating Earth's surface conditions and redox state. In present-day marine systems, microbial sulfur cycling is intimately linked with the cycling of other elements including carbon and nitrogen. The distributions of sulfur isotopes can be used as a proxy for the oxidation state of the early Earth, and can also be used to reconstruct the likely appearance and importance of various sulfur-cycling metabolic pathways. For example, the emergence of microbial sulfate reduction has been dated to at least 3.5 billion years ago (Ga), while sulfur disproportionation may only have become dominant in the mid- or late Proterozoic. However, these records suffer from ambiguities and preservational limitations. The sequencing revolution and the advent of metagenomics has offered a means to gain fresh insights into the distribution and phylogenetic history of sulfur metabolisms. For example, a recent study suggested that the dissimilatory sulfur reduction genes are distributed more widely across the tree of life than previously believed. Here, we further examine the early evolutionary history of the genes responsible for dissimilatory sulfur cycling. We infer evolutionary events including gene birth, horizontal gene transfer, gene duplication, and gene loss by reconciling phylogenetic trees of relevant genes with a time calibrated tree of life. Previous work using these methods for nitrogen cycling genes was consistent with geochemical evidence showing an early rise of nitrogen fixation and showed an increase in the proliferation of genes related to nitrite metabolism around the time of the Mesoproterozoic rise in oxygen levels. In this study, we apply these methods to dissimilatory sulfur cycling, with the aim of better constraining when distinct sulfur metabolisms arose and spread across the tree of life. These results, when combined with existing geochemical data, will provide important constraints for understanding the evolution of the microbial sulfur cycle over time.